This invention relates to a frequency shift keying demodulator, and more particularly to a modulated signal discrimination system used in a frequency shift keying demodulator having a digital phase locked loop circuit or a monostable multivibrator.
The term "frequency shift keying" implies a modulation process for transmitting data by employing different frequencies in accordance with the type of data to be transmitted and is abbreviated as "FSK". For example, for binary data having binary values of ONE and ZERO respectively, binary ONE data is transmitted by a high frequency signal while binary ZERO data is transmitted by a low frequency signal. Thus, the transmitted signal is formed of high frequency signal portions alternating with low frequency signal portions and is converted to corresponding rectangular waveforms on the receiving side. The receiving side is provided with a phase locked loop (which is abbreviated "PPL") for demodulating the received rectangular waveforms to the original data having values of binary ONE and ZERO respectively.
In a conventional PLL, a received FSK input signal has been matched in impedance and limited in bandwidth by a combined line transformer and receiver filter circuit and is adjusted so as to have a predetermined constant amplitude by means of an automatic gain control circuit (which is abbreviated "AGC"). Subsequently, an output from the AGC circuit has been supplied to a limiter where it is shaped into rectangular waveforms having transitions at zero crossings of the output of the AGC circuit. The shaped rectangular waveforms have been supplied to a digital phase locked loop circuit (which is abbreviated "DPLL") where the rectangular waveforms are converted to an output signal having two duty factors as determined by a high and a low frequency of the FSK input signal. Then, the output signal from the DPLL has been supplied to a base-band low-pass filter where it is converted to a base band signal with its high frequency component cut off after which the base band signal is sliced at a predetermined constant threshold by a threshold decision circuit resulting in a demodulated data signal.
Conventional FSK demodulators, such as that described above, have been disadvantageous in that the base band low-pass filter must be of a high quality and must have an excellent cutoff characteristic and the threshold decision circuit is unstable due to drifts resulting from variations in temperature and source voltage and is therefore required to be adjusted.